Tomodensitometry is a technique which renders it possible to obtain images of the inside of the human body by means of electromagnetic radiation transmitted through a plane section of the body examined. In effect, a densitometer comprises three principal parts:
a stand which surrounds the section which is to be depicted, and which carries means of obtaining tomodensitometric data and driving means for causing the data acquisition means to revolve around the section, PA1 a system for processing tomodensitometric data to convert the data into exploitable image data, PA1 a display unit which may for example comprise a graphic computer for image processing and a storage system.
The acquisition of densitometric data is performed at each rotation of the detection means in order to obtain a projection of the section in a given angular direction. A set of projections is obtained by means of successive rotations. The tomodensitometric data consist in the totality of the values of the projections. It is known that it is sufficient to be conversant with the totality of these projections to establish the value of the image projected, at any point. The image values are deducted from the projection values by means of a calculation in accordance with a known algorithm.
One particular design of tomodensitometer is a tomodensitometer comprising multidetectors and a source which are rotated in combination. In the case of X-rays, a source of X-ray transmits a fan-shaped beam which is intercepted by the section examined and then by a multidetector receiver which intercepts the beam as a whole. The source and receiver are fixed on a stand. Each projection thus comprises, for an angular position, the values of the measurements taken at each of the detectors of the receiver. Then, during a rotation through an angle of at least half a revolution plus the aperture of the fan-shaped beam, a theoretically sufficient coverage is obtained of the section as a whole.
A special processing operation on each projection is required to calculate the image density value at each image point ("pixel" or image element). In a particular processing example, as is known in the prior art, this processing operation comprises a linear straightening and an expansion, this latter operation corresponding to a calculation of the contributions of each projection at each image element, then to an accumulation of the contributions at each dot.
An image is available in digital form at the end of the reconstruction process; the display assembly enables making the image available to the user under visual form. Data means enable moreover image processing to be effected.
These various operations are effected at very high speed. However, one element bridles considerably the speed of acquisition of an image. In effect, the acquisition of tomodensitometric data is mechanical at the level of its collection. The periods of acquisition are therefore comparatively long. In particular, they do not permit images to be produced in the dynamic mode, that is to say sufficiently frequent to provide overall information free of interference by organs in motion or else useful information on the displacements themselves.
Partial solutions consist in reducing the periods of acquisition by reducing the amplitude of the rotations. For example, it has been proposed to couple several sources distributed uniformly on the circular stand. The gap between two sources is occupied by a receiver. A rotation having a smaller amplitude than the angle at the centre separating two sources, is sufficient. Since this angle may be limited to a few degrees, the rotation is very small and the period of acquisition very short.
A solution of this kind causes a considerable increase in the quantity of equipment however. The cost of the apparatus and of its maintenance, as well as the complication of its control system, are increased.
Another solution consists in decomposing a set of projections taken over a total angle exceeding the minimum required into several sets of projections comprising common elements, each corresponding to the minimum angular amplitude. The solution is advantageous with machines revolving continuously.
When the projection data have been acquired over almost a whole revolution, it is apparent that several different images of the same section may be reconstructed at different dates by regrouping a sufficient number of projections for each image. The period which separates two images is at least equal to the period of a rotation of the acquisition array from the position of acquisition of the first projection of the first image to the position of acquisition of the first projection of the second image.
It is thus possible to obtain two images of a phenomenon which are very close to each other in time (at best of the order of 10 ms). However, in the prior art, what is required for application of this process are either calculation means for each image for a reconstruction in real time, or else storage of all the projections and subsequent and successive processing of each image.
The first solution initially implies an increase in the number of processing systems, one only being allocated to each image, which is not actually practicable for economic reasons.
The second solution necessitates the availability of a high storage capacity for the projections and supplies the images in delayed time only. On the other hand, it imposes interruption of the acquisition once the last image of the variable phenomenon has been acquired. This is disadvantageous in the case of a tomodensitometer which operates without interruption and which successively establishes several sections.